Evolved Gas Analysis by Mass Spectrometry
Mass Spectrometry is an ideally suited technique for gas analysis due to its universality (it can detect all species, either molecular or atomic) and its very high sensitivity, with detection limits in the parts-per-million (ppm) or sometimes in the parts-per-billion (ppb) range. In our lab we can couple a quadrupole mass spectrometer to analyze the evolved gases from any of our thermal analyzers. It is more common, however, to do evolved gas analysis for thermal analyzers which measure mass change with a thermobalance (TGA or simultaneous TGA/DSC instruments) and then correlate the evolved gases with the mass change steps in the TG curve. But it is also possible, and sometimes very useful, to correlate the evolved gases with the DSC peaks. If a peak or feature in the DSC curve is not associated with changes in the gas atmosphere, then it can be reliably assigned to a phase transition, e.g. Curie transition in magnetic materials or melting and crystallization events.
We couple the quadrupole mass spectrometer to our thermal analyzers via a heated fused silica capillary which brings a small portion of the gas from the furnace outlet to the mass spectrometer for analysis. This set up is very robust and works well mostly for permanent gases and liquids with relatively low boiling points. The figure below shows the TG curve for calcium oxalate in inert atmosphere overlaid with the MS ion-current curves. Water can be easily identified from its parent signal at 18 amu and the OH fragment at 17 amu, and it is clear that the first mass loss step in the TG curve is due to the loss of hydration water. Evolution of CO (28 amu signal) and CO2 (44 amu) are associated with the second and third mass loss steps, respectively. In oxidizing atmosphere, there will be only CO2 evolved in the second and third mass loss steps, which respresent the decomposition of anhydrous calcium oxalate to calcium carbonate, and, subsequently, the decomposition of calcium carbonate to calcium oxide and carbon dioxide.
The analysis of heavy inorganic species such as metal vapors, volatile chlorides and other salts as well as large organic molecules is often not possible with the capillary coupling setup because the gaseous species generally will not travel all the way to the mass spectrometer, they would rather condense on the walls of the fused silica capillary. For the analysis of such species, a much more complicated and expensive setup is needed called a molecular beam (MB) mass spectrometer inlet, sometimes also referred to as skimmer coupling. This setup samples the evolved gas at a very short distance from the furnace outlet without the use of a capillary, thus ensuring that the coupling interface is almost at sample temperature, which almost completely eliminates the possibility for condensation.